Residual fuel left in a gas turbine engine fuel nozzle supply line after engine shutdown in subject to high soak back temperatures. As a result, the residual fuel tends to coke and/or clog up the fuel nozzle or coke the combustor can walls, thereby detrimenting engine performance on subsequent operation. To obviate these difficulties it is desirable to drain or dump the residual fuel from the nozzle supply line in response to engine shutdown.
Although numerous elaborate gas turbine engine fuel control systems have been devised, none have included a satisfactory nozzle supply line residual fuel dumping feature. Past attempts to accomplish drainage of the residual fuel have generally involved incorporation of the dumping feature in the nozzle fuel supply valve. In this regard, the supply valve may include a valved drain passage in addition to a valved supply passage which are operable in mutually complementary relationship. In other words, the drain passage is closed when the supply passage is open to supply fuel to the nozzle. Conversely, when the supply passage is closed to block fuel from the nozzle and shut down the engine, the drain passage is open to dump the residual fuel from the nozzle fuel line.
However, if a spool valve is employed for such a dual function, leakage to the drain passage is too great at the high fuel pressures which exist during normal engine operation. If a poppet valve is employed, a relatively large solenoid is required to pull the valve into position against the high fuel pressures. Hence, prior art approaches to drain the nozzle fuel line upon shutdown of a gas turbine engine have been unsatisfactory from the standpoint of the excessive fuel leakage experienced during normal engine operation and/or the cost of providing the fuel dumping feature, among other things.